This application claims the priority of German patent document 198 46 319.7, filed Oct. 8, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an energy supply circuit for a motor vehicle on-board electrical system having two voltage supply branches.
The development of new components in automobile construction, such as the electromagnetic valve control (EMVC) of the electrically heatable catalyst, etc., as well as the trend toward electrically driven components (which heretofore had been driven by way of belts), has considerably increased the total electric power of the consuming devices to be supplied. This power demand can no longer be reasonably satisfied by means of the current 12V electrical systems on the basis of 14V generator voltage. It is known to superimpose higher voltage levels on the 12V on-board electrical system, which feed the high-load consuming devices, such as the EMVC, the heaters, fans, servo motors, etc. Consuming devices with a lower power consumption remain at the 12V level.
It has increasingly been found that an on-board electrical system with a 42V voltage supply is desirable for high-load consuming devices, and a 12V or 14V voltage supply is desirable for low-voltage consuming devices, such as the lighting or electronic control units. The voltage level of the high-load consuming devices is thereby raised to three times the current 14V level (generator voltage).
An on-board electrical system arrangement with two batteries which can be energetically coupled is disclosed in German Patent Documents DE 40 28 242 A1 and DE 38 41 769 C1, in which the batteries are approximately at the same voltage level of 12-14V. In addition, an on-board electrical system with two voltage levels is known from German Patent Document DE 196 00 074 A1, in which the higher voltage level is implemented by a parallel connection of several chopper stages.
In a typical configuration which is known per se and which is illustrated in FIG. 3, a generator G buffered by means of an energy accumulator B2 feeds a starter S and a 42V system for the high-load consuming devices HV. The 14V system of the low-load consuming devices LV is supplied by way of an electronic power system connected on the input side with the generator G, for example, by way of a DC/DC converter W, such as a unidirectional buck converter or a bidirectional buck-boost converter. The 14V system is also buffered by means of an energy accumulatorxe2x80x94battery B1xe2x80x94for emitting power peaks and against surge voltages.
The raising of the voltage level of the high-load consuming devices to 42V achieves several advantages. Rectifier losses in the generator are reduced to one third. The reduction of the currents while the power is the same permits a decrease of cable cross-sections, and facilitates the use of semiconductor switches, the replacement of screw-type connectors by plug-type connectors, etc. The relative voltage drop and the mass offset are also decreased. Additional advantages and advantageous methods of operation of the known on-board system architecture will be outlined in the following.
It is considered to be a disadvantage of the above-described on-board system configuration that the voltage limits to be specified for the 14V branch of the low-voltage consuming devices LV must comprise the full voltage level difference which occurs as a result of both the required charging voltage, on the one hand, and the discharging voltage during buffering operations for covering peak performances or the supply of consuming devices when the engine has stopped, on the other hand. Thus, for example, when a conventional lead acid battery is used for the consuming devices, a tolerance band of 11-16V is required. When other types of batteries, such as NiMH nickel metal hydride batteries, are used, a still wider tolerance band may be required because of different ratios of the end-of-charging voltage to the cell end voltage.
The voltage level difference occurring in the electrical system makes special demands on the design of the consuming devices, increases costs and may have negative effects, such as lifetime reductions in the case of bulbs.
It is an object of the invention to provide an improved on-board electrical system architecture, while maintaining the advantages of the conventional systems described above, such that a narrower tolerance band is permitted for the consuming devices.
These and other objects and advantages are achieved by the energy supply circuit according to the invention which has two voltage supply branches at different voltage levels. The first voltage supply branch can be fed by way of an electric direct voltage converter by the second voltage supply branch and the second voltage supply branch can be fed by a generator. A multi-level controller W3 with three voltage levels is provided whose one in/output is connected with the second voltage supply branch, whose second in/output is connected with the first voltage supply branch and whose third in/output is connected with the energy accumulator B1 assigned to the first voltage supply branch.
According to the invention, the low-voltage system (first voltage supply branch) is no longer directly connected with the energy accumulator B1 assigned to it but by way of a multi-level controller W3. The uncoupling of the supply of the low-voltage system from the battery terminal voltage makes it possible to control the output voltage of the first converter W1, which feeds the system of the low-voltage consuming devices LV, to a lower, narrow tolerance discharge voltage U_E, which corresponds to the discharge voltage of the battery. As a result, a narrow tolerance band (such as 11.8V . . . 12.8V) of the supply voltage can be defined, which facilitates the design and, for example in the case of bulbs, prolongs their useful life.
In a further embodiment of the invention, the uncoupling permits optimal rapid battery charging with a charging voltage U_L adapted to the temperature of the battery, without undesirable effects on the low-voltage consuming devices LV in the first voltage supply branch, which are fed with the lower, narrow-tolerance discharge voltage U_E.
As a function of the voltage ratios in the on-board electrical system, the multi-level controller W3 can be switched into additional advantageous operating modes.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.